This invention relates to a rotational molding of powdered polyolefins. In one aspect, it relates to a method of preparing granular resin for use in rotational molding of plastic products. In another aspect, the invention relates to a method for shaping of and incorporating additives onto rotational molding resin.
In one type of rotational molding, a granular polymer is placed in a mold and heated while rotating the mold. The granular polymer melts as the temperature increases forming a homogeneous layer of uniform thickness on the mold walls. Following cooling of the mold to solidify the polymer, the mold is opened and the finished product removed.
Rotational molding resins should satisfy the following criteria:
1. The granules must be free flowing in order to permit charging to the mold and conforming to the mold configuration. PA1 2. The granules should be substantially spherical in shape and free of any tails or hairs which could interfere with the flowability of the particles. PA1 3. The particle size should be relatively small and the particle size distribution of the granules should be relatively narrow (less than 5.0 weight percent larger than about 30 mesh and only minor amounts (less than 15%) finer than 100 mesh. PA1 4. The bulk density of the granules should be high to provide good flowability, close compaction in the mold, and reduce shipping costs. PA1 5. The additives (e.g. antioxidants, UV stabilizers, etc.) should be thoroughly dispersed in the granules and preferably in contact with all granules because no mixing occurs during molding.
The principal prior art technique for preparing resin for rotational molding involves the following two steps: (1) The resin is pelletized during which additives are melt compounded and (2) The pellets are ground and classified to the proper particle size. The pelletizing and grinding operations not only adds to the expense due to increased energy requirements, but frequently produces irregularly shaped particles or fragments having hairs of tails which impede flowability and reduce bulk density. Moreover, the melt blending step carried out at elevated temperatures can result in poor end use performance of the product, production or polymer degradation. As mentioned previously, all of these factors can contribute to a poor quality rotomolding product.